1. Field of the Invention
The present invention relates to a developing device used to form an image by employing an electrophotographic method or an electrostatic recording method. More specifically, the present invention relates to a developing device that transfers a developer between a plurality of developer bearing members.
2. Description of the Related Art
To date, many developing devices equipped with a developing sleeve have been proposed and employed, as developing devices for utilizing an electrophotographic method or electrostatic recording method to develop an electrostatic latent image formed on an image bearing member with a single-component developer or a two-component developer.
Typically, the developing sleeve is rotatably supported by an opening of the developing device through bearings arranged at both ends of the developing sleeve. The developing sleeve is subjected to a surface roughening processing through, for example, blasting. This developing sleeve bears and conveys a developer, and visualizes a latent image on an image bearing member with the borne developer.
If the quantity of the developer on the surface of the developing sleeve is not uniform, the density of the visualized image on a photosensitive member may also become non-uniform, in which case a problem with an image quality arises. To avoid this problem, it is desirable to equalize the quantity of the developer on the surface of the developing sleeve. In general, use of a regulating member called a regulating blade permits the quantity of the developer on the surface of a developing sleeve to be uniform.
A typical developing device is equipped with a development container that stores a developer, and conveyance members, such as a screw, disposed in the development container. These conveyance members circulate and convey the developer within the development container.
On the other hand, recently, the operating speed of such an image forming apparatus using the electrophotographic method has been increased. Japanese Patent Application Laid-Open Nos. 2003-323052, 2007-72221, and 60-061776 discuss developing devices that are operable at a high speed. These developing devices are each equipped with a plurality of developing sleeves using two-component magnetic brushes, to increase development opportunities. The developing device in Japanese Patent Application Laid-Open No. 2003-323052 includes an upstream developing sleeve provided with a blade, and a downstream developing sleeve positioned below the upstream developing sleeve, as illustrated in FIG. 1, and both of the sleeves transfer and convey a developer while rotating in the same direction. Thus, this developing device yields the compactness despite employing the twin sleeves.
As for twin developing sleeves using, as described above, a two-component developer, Japanese Patent Application Laid-Open No. 2007-72221 discusses a technique for suppressing a developer from staying between upstream and downstream developing sleeves by specifying the magnetic force between the respective transfer poles in the upstream and downstream developing sleeves, as illustrated in FIGS. 11A, 11B, and 11C.
Unfortunately, the recent increase in the operating speed of developing devices has caused the following problems. The surfaces of the sleeves which have undergone the blasting processing are likely to be worn out along with the continuous use. In such a case, the developer may slip over the surfaces of the sleeves rotating at a high speed, thereby causing the degradation of the transfer efficiency. In addition, the developer that stays between the sleeves may accelerate the chipping of the sleeves. To secure the conveyance capacity for continuous use, a groove sleeve is employed, in which a plurality of grooves for conveying a developer is provided on the outer surface of each developing sleeve. In Japanese Patent Application Laid-Open No. 60-061776 that is a patent document about the groove sleeve in the twin developing method, all the grooves in the downstream developing sleeve have a larger total volume than those in the upstream developing sleeve, as illustrated in FIGS. 12A and 12B. This structure relatively increases the conveying capacity of the downstream developing sleeve, thereby suppressing the developer from staying between the sleeves.
Even by setting the total volume of all the grooves in the downstream developing sleeve larger than that in the upstream developing sleeve as in the conventional way, however, the transfer efficiency of each groove cannot be increased effectively. In fact, there arises a risk that, when the upstream developing sleeve transfers the developer to the downstream developing sleeve, the developer staying between the sleeves is degraded by being rubbed therebetween. For example, when the downstream developing sleeve having an increased number of grooves is used to increase the transfer efficiency, a large number of grooves may pass through the developer staying between the upstream and downstream developing sleeves, in which case the developer could be degraded. The degraded toner accumulated in the grooves of the sleeves may be contaminated, thereby degrading the conveyance capacity. To reserve the conveyance capacity even if the toner contaminated to some extent is present in the grooves, a configuration employing deep grooves is conceivable. However, if excessively deep grooves are used, a co-rotation phenomenon may occur, that is, the developer which has not been chipped by the repulsive poles in the downstream developing sleeve may be discharged from the gap between the upstream and downstream developing sleeves. The occurrence of the co-rotation phenomenon may result in the degradation of the transfer efficiency.